The present invention relates to an improved means and method of lapping thin wafer like materials, and more particularly to the use of sound transmission during the lapping process.
The preparation of semiconductor devices made from silicon, gallium-arsenide, and the like, begins with growing a monocrystalline boule. The boule is then sliced into thin disks called wafers. The wafers are then circumferentially ground, lapped, chemically etched, polished, and cleaned.
The lapping step is conventionally carried out in large orbital lapping machines that are well known in the art.
In the lapping process, the wafers are positioned in circular plates called carriers. The carriers are designed with holes prepared in them that are slightly larger in diameter than the wafer to be lapped, and are slightly thinner than the target thickness of the wafers at the end of the lap cycle. The carriers are circular shaped, and usually have gear teeth around the outside periphery. The gear teeth interact with a center gear and an annular gear such that a drive motor turning the gears causes the carrier to rotate around the center gear and around itself in an orbital motion. The design of the carrier positions the wafer-containing holes toward the outer edge of the carrier such that they rotate around the center of the carrier itself during machine operation. By changing the gear ratios of the carriers, annular gear, and center gear, the rotational direction of the carriers can be controlled and changed as needed to control flatness and wear of lapping plates.
Both the carriers, and the wafers contained within them, are supported by a bottom lapping plate. Upon starting a lap cycle, a top lapping plate lowers onto the wafers under a controlled, and typically very light, pressure. A slurry containing an abrasive and other components such as a soap, rust inhibitor, or surfactant as desired, is introduced to the wafers through slurry supply holes in the top plate. One or both of the plates are typically linked to either the center gear or the annular gear such that they rotate in a controlled ratio and direction along with the center gear, annular gear, and carrier. The combined rotation of each of these items results in the wafers moving in a circular motion within the carrier, around the carrier, and around the center gear, with the top and bottom lapping plates rubbing against the two flat surfaces of the wafer. Wafers lapped in such a manner have very smooth and flat surfaces, with a high degree of uniformity between wafers lapped in the same cycle.
Following an operator-designated time, additional pressure is gradually exerted on the wafers from the top plate either from the weight of the top plate or from a mechanical means. The force exerted on the wafers, the motion of the wafers around the plates, and the abrasive in the slurry combine to lap away the surfaces of the wafer in small increments. When the desired thickness of the wafers is reached, the lap cycle is stopped, slurry delivery is stopped, and the top plate raises off the wafers. The wafers are then removed from the lap machine.
The lapping plates used for the process are typically metal, and have grooves cut into the surfaces that are in contact with the wafers. Among other functions, the grooves assist in supplying slurry to the entire surface of the wafers, carriers, and plates, and to facilitate the removal of the used slurry and residue of the lapped wafer. The grooves can be cut into the plates in many different patterns as the user desires. The grooves are typically about 5 millimeters wide, and initially about 15 millimeters deep. Over time however, the surfaces of the lapping plates are abraded away, and the grooves become much more shallow. As a result, the lapping plates either need to be replaced, or resurfaced with new grooves cut into the surfaces.
As the lap machine is used in repeated cycles, the grooves become clogged with the residue slurry and the material lapped from the wafers, plates, and carriers, inhibit removal of the waste slurry and become a trap for particles and contaminants. The grooves, when clogged, create a buildup of slurry on the bottom plate that causes the wafers to float during the start of the lap cycle. Wafers can then escape the confines of the wafer-containing holes of the carrier, and break in the lap machine. Wafer shards can then become lodged in the residue slurry in the grooves of the bottom plate and become projectiles that can scratch wafers lapped in future cycles.
Wafers processed in the lap machine typically have a beveled circumferential edge. As previously mentioned, the diameter of the wafer-containing holes in the carrier is only slightly larger than the diameter of the wafers being lapped. These conditions combined make it possible for the operator to improperly seat the wafer into the wafer-containing hole such that the beveled edge of the wafer rests on the edge of the carrier. In such an instance, when the top plate is lowered onto the improperly seated wafer, it will break. As previously indicated, a broken wafer in the machine causes significant damage to other wafers, as well as to the lap machine, and may result in extended machine downtime. Any scenario involving a broken wafer in a lap machine is referred to as a xe2x80x9ccrashxe2x80x9d.
To prevent a crash, the operator will listen very closely to the lap machine during the first minute or more of the lap cycle. Typically, when a machine is about to crash, a distinguishable chatter-noise is heard. When the operator hears that particular noise, the machine is immediately stopped and the wafers are re-seated in the wafer-containing holes. The lap cycle is then restarted.
In an effort to reduce production costs, semiconductor manufacturers are continually striving for larger diameter wafers. As the diameter of wafers increases, new and larger processing equipment must be designed and implemented. The design of late model lapping machines makes it very difficult for the operator to hear chatter-noise originating from the center or back of the machine. The newer designs also incorporate a plastic safety shield that encloses the front of the machine, which further inhibits the operator being able to detect a crash by hearing the chatter-noise.
Consequently, there is a need of an apparatus to assist in the early detection of chatter-noises symbolic of a lapping crash wherein wafers leave the confines of the wafer-containing holes in the carriers and break in the lapping machine.
The apparatus disclosed herein includes a sound transmitting device, such as a microphone, placed within the confines of the safety shield, and oriented in such a way as to capture and transmit sounds generated within the lapping machine. The apparatus also includes a receiver placed outside the confines of the safety shield in a location convenient for the operator that receives and broadcasts sounds generated within the lapping machine. The apparatus may include an amplifier to increase the power of the sound output in any or all frequency ranges such that an operator may selectively screen for particular sounds.